Production of molded articles from polytetrafluoroethylene



July 15, 1952 s, T DEAKlN 2,602,962

PRODUCTION OF MOLDED ARTICLES FROM POLYTETRAFLUOROETHYLENE Filed March 25. 1949 2 SHEETS.SHEET 2 'INVENTOR STANLEY THOMAS DEAKIN Patented July 15, 1952 PRGDUCTION F MOLDED ARTICLES FBQM POLYTETRAFLUOROETHYLENE Stanley Thomas Deakin, Ewell West, England, assignor to British Mechanical Productions Limited, London, England Application March 25, 1949, Serial No. 83,307 In Great Britain March 25, 1948 Claims. 1

The invention relates to the production by moulding or forming of articles of intricate shape from plastic materials (e. g., poly-tetrafiuorethylene) of the kind which at normal temperatures are rigid or pliable but cannot be satisfactorily moulded and which at elevated temperatures become capable of a high degree of elastic deformation.

The invention provides the method of producing moulded or formed articles from plastics of the above kind which comprises the steps of heating a pellet of the material to a temperature at which it is elastic, moulding or forming the elastic material to the desired form and coolene a tool temperature of l60 to 200 C. is found to give the best results.

Poly-tetrafluorethylene has a high coefiicient of expansion up to the temperature (about 327 C.) at which it becomes elastic and above that temperature it has a still higher coeficient of expansion. It is therefore necessary, in many cases, in order to obtain properly formed mouldings, to maintain moulding pressure on the material in the mould during cooling (at least until the temperature has fallen to about 327 C. and preferably until the temperature of the article has fallen to approximately that of the mould) to follow up shrinkage of the material. This feature of maintaining the pressure constitutes an important part of the method according to the invention. It is also necessary to prepare the mould with sufficient allowance in dimensions to accommodate the shrinkage of the material from the temperature at which the moulding pressure ceases to be effective. This temperature which may be found by experiment will normally be between 327" C. and the temperature at which the mould operates.

The pressure employed in carrying out the invention is comparatively low in relation to the pressures normally employed in the moulding of thermo-setting plastics and will be considerably less than 1,000 pounds per square inch.. L

In view of the high coefiicient of expansion of the moulding material it i important thatthe pellet dimensions be chosen to allow for thegex pansion with heating and thereby to ensure that the hot pellet may readily be inserted into the mould.

The invention may be employed in the production of many items of electrical equipment and more especially radio equipment. 1

Specific applications of the methodaccording to the invention to the production of a simple moulding from poly-tetrafluorethylene and to the production of a socket holder for a radio valve holder will now be described by way of example and further description of the invention, with reference to the accompanying drawings in which: Figure 1 is a section on the line I l in Figure 2 of a moulding tool for use in'the production at one operation of four washers; Figure 2 is a plan view of the lower half of-the moulding tool;

Figure 3 is a view of one of the washers; Figure 4 is a diagram showing the means for heating the pellets and for feeding them to'th'e mould;

Figure 5 is a plan of the socket holder; Figure 6 is an under plan of the socket holder"; Figure 7 is a section on the line 7-1 in Figure 6 showing one of the socket elements in position.

The washers produced according to the first application of the method are required to be of precise internal and external diameters to makefla close sliding fit'on a rod and inside a cylinder and cannot be produced of sufiicient dimensional accuracy by compacting and sintering. 1' The washers are accordingly produced by'moulding by the method of the invention. The pellets used to produce the washers are of simple disc form. The pellets, one for each moulding, are prepared by weighing outthe' appropriate quantity of powdered poly-tetrafluorethylene for one pellet, loading the quantity into a mould shaped to produce a pellet of a size and shape suitable for insertion, when hot, into the final mould, and then compacting the powder, while cold, under a pressure of about 1,000 lbs/sq. in. The weight of material required in each pellet may be determined approximately, by measuring out a volume of the powdered material equal to the volume of the final mould multiplied by a factor (usually about 3.5 but varying according to the fineness of the powder) for the four female moulds 2 and the upper part 3 has i four mould plungers 4 aligned with the female 10.

moulds. The upper part is supported on guide pillars 5 and its downward movement is limited by stop-collars 6 on the pillars. Both parts of the mould are provided with thermostatically controlled electric heating elements shown diagrammatically at 8 and each part has a heat insulating pad 9. 4

The female moulds and the plungers are shaped to co-operate to produce washers of the required form, shown in Figure 3, and the plungers have axially projecting pins It to. form. the central holes in thewashers- Means-for heating the pellets and transferring them to the moulds are showndiagrammatically lhzFlgureA; The pellets are loaded: into sockets llin heat-retaimng carriers I 6. The carrierspass through an electrically heateditunneloven l7 and are fed one at a time onto the lower part. I' of the mould. Eachcarrier. has a slide l8- normally forming a bottom to thesockets. 15. The slide has four holes and-when thefour sockets are'positfoned over the female moulds the. slide is moved toialign' these holes. with. the sockets; to permit the -pellets1tofall into the moulds. The plungers then operate'through thesockets;

Thepelle'ts are heated in' the oven" to a temperatureof 327-450"C. (depending upon' the; size ofthepellet) at" which temperature the compacted powder sinters into a homogeneous mass. The mould parts. are. heated. to a temperature of I60 200C; and are maintained within that temperature range. The moulding pressureis maintallied for a period'of 0.5 to 1 minute (depending upon the size of the article) during which time the material cools down to a temperature at which it has lost. its elasticity. The moulding isthen removed.

} Figures 5-7 show a typical form ofsocket holder whichmay bemoulded from poly-tetrafiuorethylene' by: the method according to the invention. The holder-is similiar to that described imBritish patent-specification No. 618,677. In the holder shown, however, the slots. are made a close fit around the. stems. 2i of the-socket elements andv the. retaining lugs 22 are forced through the slots, the material. evencat normal temperatures being sufficiently elastic for this purpose.

The pellets used in the. production. of the holders are in the form of a short cylinder with an axial throughway and, in one case, weighed about 3.2 grammes each The time taken for the moulding operation in this case was about 30 seconds.

It is to be appreciated that the method provided by the present invention employs the principle of heating the material until it becomes elastic, deforming the elastic material and then freezing the material in the deformed condition.

I claim:

1. The method of moulding articles of polytetrafluorethylene which comprises the steps of heating a. pellet of the material to a temperature above 327 C., transferring the heated pellet to a mould'which is at a temperature of between andv 200? CI, deforming the pellet by pressure to theshape of the'mould and cooling the material in themould to'a temperature below 327 C. while maintaining pressure on the material and while maintainingqthe mould at a temperature between 160 and 200 C.

2. The method according to claim 1 in which the pellet is heated to a temperature between 327 C; and..450.-C.

3. Themethod according to claimland including; the initial steps of compacting powdered polytetrafluorethylene into the form ofv the pellet and then. sintering the material.

4. The method of moulding articles of polytetrafluorethylene which comprises the steps. of heating a pellet of the material to a temperature above 327 C., transferring the heated pellet to a mould whichis. maintained at a temperature of between about 160 and 200 C., deforming the pellet. by pressure to the shape of..the. mould, cooling. the material. inthe. mould to a temperature between about 1.60and 327C., allthewhilemaintaining the mould at. a temperature, between about lfilland 200 C. and maintining pressure on the material.

5. The method of moulding articles of polytetrafluorethylene which comprises the steps of heating a pellet of the material to a temperature above 327 C., transferring the heated pellet to a mould which is maintained at a temperature of between about 160 and 200 C., deforming the pellet by pressure to the shape of the mould, cooling the material in the mould to a temperature between about 160 and 327 C., all the while maintainingthe mould at a temperature between about 160 and 200 C. and maintaining pressure on the material, and removing the object from'the mould and cooling it to room temperature.

STANLEY THOMAS DEAKINY.

REFERENCES CITED The following references. are of recordin the file of this patent:

UNITED STATES PATENTS Number Name Date 2,379,247 Muskat June 26, 1945 2,432,668 Kingston Dec. 16, 1947 2,433,643 Beach et al Dec. 30, 1947 2,456,262 Fields Dec. 14, 1948 

1. THE METHOD OF MOULDING ARTICLES OF POLYTETRAFLUORETHYLENE WHICH COMPRISES THE STEPS OF HEATING A PELLET OF THE MATERIAL TO A TEMPERATURE ABOVE 327* C., TRANSFERRING THE HEATED PELLET TO A MOULD WHICH IS AT A TEMPERATRE OF BETWEEN 160 AND 200* C., DEFORMING THE PELLET BY PRESSURE TO THE SHAPE OF THE MOULD AND COOLING THE MATERIAL IN THE MOLD TO A TEMPERATURE BELOW 327* C. WHILE MAINTAINING PRESSURE ON THE MATERIAL AND WHLE MAINTAINING THE MOULD AT A TEMPERATURE BETWEEN 160 AND 200* C. 